Aircraft control system

ABSTRACT

This invention control system having: means ( 12 ) for detecting an irregularity in aircraft handling during flight; means ( 18 ) for causing temporarily a rigid body excitation in at least a portion of the aircraft; means ( 20 ) for monitoring an actual response to the excitation; means ( 22 ) for comparing the actual response and a target response to the rigid body excitation; and means ( 14 ) responsive to an output from the comparison means for determining the need for a modification of the current flight plan and for generating a corresponding control output. The invention also provides a corresponding method for controlling an aircraft.

This application is the U.S. national phase of international applicationPCT/GB01/01003, filed in English on 8 Mar. 2001 which designated theU.S. PCT/GB01/01003 claims priority to GB Application No. 0007619.0filed 29 Mar. 2000. The entire contents of these applications areincorporated herein by reference.

The present invention concerns an aircraft control system, primarily foran unmanned aircraft for enhancing or otherwise improving theairworthiness of the aircraft.

Unmanned aircraft, even when remotely piloted, may still encountersituations when they have to operate with a degree of autonomy and inthe absence of communication between the aircraft and ground. Such asituation may arise, for example, in the event of the communicationdifficulties between the aircraft and the control station or onoccasions when flight stealth is required.

Flight accuracy and safety during times when an unmanned aircraft is notin communication with a ground control station is a high requirement,but one which has not yet been satisfactorily resolved.

In a manned aircraft, the pilot receives positive indications of theaircraft's status from the sensor, safety and control systems of theaircraft and from the various systems failure warnings, and he usesthese signals for responding to the current flight circumstances. Inaddition, the pilot also has an “intuitive sense” as to when hisaircraft is behaving in an unusual or potentially dangerous manner.These additional flying skills are not available to the ground basedremote operator of an unmanned aircraft, especially in circumstanceswhen the unmanned aircraft is out of communication with the operator.This may be reflected in the responses of the unmanned aircraft tounexpected flight conditions, and hence in the accuracy and safety ofthe aircraft flight.

It is an aim of the present invention to improve the ability of anunmanned aircraft to detect, locate the cause of and take correctiveaction for unexpected or unusual circumstances arising during flight.

According to the present invention, an aircraft control systemcomprises:

-   -   means for detecting an irregularity in aircraft handling during        flight;    -   means responsive to an output from the detecting means for        causing a rigid body excitation in at least a portion of the        aircraft;    -   means for monitoring an actual response of the aircraft to the        rigid body excitation;    -   means for effecting a comparison of the actual response with a        target response of the aircraft to the rigid body excitation;        and    -   means responsive to an output from the comparison means for        determining the need for a modification of the current flight        plan for dealing with the irregularity and for generating a        corresponding control output.

The comparison means may include means for generating a flight conditionanalysis, and the determining means may be arranged to evaluate thecurrent flight plan on the basis of such flight condition analysis inorder to generate a control output in the form of a flight controlsignal for effecting corrective action if necessary.

In a preferred embodiment of the invention, the means for causing arigid body excitation in at least a portion of the aircraft are arrangedtemporarily to apply a low level periodic exciting force to a rigidframe portion of the aircraft. In this instance, the monitoring meansmay be arranged to detect a transient response of the rigid frameportion of the aircraft to such excitation. The actual response of andthe target response for the rigid frame portion may then be compared interms of frequency/transient characteristics.

For example, the excitation may be applied to a rigid frame portion ofthe aircraft by briefly imposing a low amplitude periodic motion on topof the normal movement of the flight control surfaces of the aircraft.This periodic motion may conveniently be sinusoidal.

In a preferred form of the invention, means are provided for calculatingthe target response to the rigid body excitation, in which case thecalculation means may employ a complex mathematical model forcalculating the target response. For example, the aircraft may have astored computer model of expected responses based on the Euler DynamicEquations of Motion and established during the aircraft design process.

More particularly, the mathematical model may be based on an analysisduring design of the combined control laws and aircraft dynamiccharacteristics, including anticipated frequency, transient and dampingresponses for the specific design criteria of the aircraft.

In a preferred form of the invention, the means for monitoring theactual aircraft response includes means for measuring inertial aircraftbody rates, accelerations, and control surface positions.

According to another aspect of the present invention, there is provideda method of aircraft control comprising:

-   -   detecting an irregularity in aircraft handling during flight;    -   causing a rigid body excitation in at least a portion of the        aircraft in response to such detection;    -   monitoring an actual response of the aircraft to the rigid body        excitation;    -   comparing the actual response with a target response of the        aircraft to the rigid body excitation; and    -   determining the need for a modification of the current flight        plan in response to such comparison and generating a        corresponding control output.

The invention is described further, by way of example, with reference tothe accompanying drawings in which:

FIG. 1 is a block diagram of a conventional flight control system foruse in an unmanned aircraft;

FIG. 2 is a block diagram of a modification of the flight control systemof FIG. 1 according to the present invention; and

FIG. 3 is a flow diagram representing operation of an aircraft computerof the flight control system according to the present invention.

Referring initially to FIG. 1, an unmanned aircraft conventionally has aflight control system 10 including a plurality of sensors 12 fordetecting current flight conditions, such as aircraft velocity, aircraftacceleration, aircraft attitude, air data, control surface positions,steering commands etc. The sensors 12 provide output signals to anonboard computer 14 for monitoring the flight situation in relation to apredetermined flight plan. The flight computer 14 then provides anoutput to control means 16 for adjusting the aircraft flight asrequired.

The flight computer 14 may also be arranged to receive signals from aremote operator at a ground control station if desired.

The control system shown in FIG. 1 operates by way of a feed-backarrangement in which the aircraft flight is monitored by the sensors 12and the flight computer 14 and then appropriate adjustments are made inthe aircraft controls and control surfaces to maintain the desiredcourse and speed.

Turning now to FIG. 2, the present invention envisages supplementing thecontrol system as shown in FIG. 1 with additional features. According tothe invention, the aircraft also includes excitation means 18 forsubjecting at least a portion of the aircraft frame temporarily to arigid body excitation in response to an indication from the sensors 12and flight computer 14 that there is an anomaly in aircraft handlingduring flight. In the present instance, it is envisaged that theexcitation means 18 will respond to such indication by briefly applyinga low level periodic exciting force to cause a low amplitude sinusoidalmotion in the flight control surfaces through the normal flight controlactuators. The low amplitude of such motion will have the advantage ofkeeping any resulting flight path deviations to a minimum.

Detection means 20 are arranged to monitor the actual response of theaircraft or portion thereof to the temporary excitation, for example bydetermining the frequency/transient characteristics of one or moreselected regions of the aircraft body to the excitation. The detectionmeans 20 may also employ data based on measured inertial aircraft bodyrates, aircraft accelerations and measured control surface positions formonitoring the response of the aircraft to the excitation. The detectionmeans 20 are arranged to generate a detection output for supply tocomparison means 22.

In addition, means 24 are connected to the excitation means 18 forcomputing a target response to the temporary rigid body excitation. Thecomputing means 24 may employ a complex mathematical model forcalculating the target response and in the present instance includes astored computer model of expected responses based on the Euler DynamicEquations of Motion. Such a computer model is established during theaircraft design process and is stored in the computing means 24 at thistime. Alternatively, if the target response does not vary with flightconditions, e.g. atmospheric pressure, then the computing means 24 maymerely be a memory, such as a look up table, storing the targetresponse.

The computing means 24 are arranged to produce an output representingthe target response for supply to the comparison means 22.

The comparison means 22 receives the outputs from the detection means 20and the calculation/storage means 24 and performs a comparison of theactual aircraft response with the target aircraft response. Thecomparison means 22 are arranged to generate a flight condition analysisrepresenting the difference between the actual aircraft response and thetarget response for supply to the flight computer 14.

According to the present invention, the flight computer 14 evaluates theflight condition analysis in relation to the current flight plan andincludes the outcome of such evaluation in its decision making processfor prompting a decision to modify and/or alter the existing flight planif necessary. Such a decision might include a modification in the formof a corrective action or adjustment within the control system, forexample a reversion to a lower level of system multiplexing, or a changein control loop gains. Alternatively, such a decision might include analteration to aircraft mission, for example an instruction to land assoon as possible, or to fly to a pre-defined safe place and ditch.

Turning to FIG. 3, this shows a flow diagram representing the operationof the flight computer 14 according to the present invention. Thisoperation will now be described.

In step 40, the computer 14 receives data from the sensors 12 in thenormal way, representing aircraft velocity, aircraft acceleration,aircraft attitude, air data, control surface positions and steeringcommands, for example. The computer 14 evaluates such data in step 42 toverify that the data is consistent and that it can be identified with aunique flight condition according to a pre-determined flight plan. Inthe event that such verification is satisfactory, the computer proceedsto step 44 in the normal way and generates a flight control output forsupply to the control means 16 for adjusting the aircraft flight inaccordance with the flight plan as required.

On the other hand, in the event that the verification of step 42indicates an anomaly or irregularity in the handling of the aircraft,then and only then the computer proceeds to step 46. By way of example,if the sensor data supplied in step 40 represents an aircraft pitch ratethat does not relate to the incidence, the normal acceleration and thetail surface angle for the particular flight condition, then theverification made in step 42 will determine that an anomaly orirregularity is present, and the computer will proceed to step 46.

In step 46, the computer will generate an output to the excitation means18 to cause a temporary exciting force to be applied to a rigid bodyportion of the aircraft, for example to cause a low amplitude periodicor sinusoidal motion in the control surfaces of the aircraft via thecontrol surface actuators.

Following this, in step 48, the computer will now receive and monitorthe sensor data obtained from the sensors 12 in step 40 and willcalculate the actual response of the aircraft to the rigid bodyexcitation.

In step 50, the computer will simultaneously calculate a set ofallowable characteristics for factors such as frequency responseboundaries, damping factors, and transient responses covering the fullflight envelope and representing an anticipated or target response tothe rigid body excitation. Data representing the target response is thencompared in step 48 with data representing the actual aircraft responseand an output in the form of a flight condition analysis is generated.

If the comparison of the data for the actual and target responses yieldsa satisfactory result, in that the actual aircraft response falls withinthe allowable characteristics calculated in step 50, then the computerproceeds to step 52 indicating that no action is required. On the otherhand, if the comparison effected in step 48 indicates that the actualaircraft response does not fall within the allowable characteristicscalculated in step 50 and that certain limits or boundaries have beenexceeded, the computer proceeds to step 54.

In step 54, the computer receives data representing flight controlsystem status based on the pre-determined flight plan, such data beinggenerated in step 56. The flight condition computer evaluates such dataanalysis together with data generated in step 48 including datarepresented the limits or boundaries that have been exceeded, anddetermines what modifications and/or alterations to the existing flightplan are necessary for dealing with the irregularity or anomaly. Thecomputer issues a decision in step 58, which may be a decision to effectadjustments in the existing flight control program, for example torevert to a lower level of system multiplexing or to change systemcontrol loop gains, or which may be a decision to alter the aircraftmission and to land as soon as possible or to fly to a predefined safeplace to ditch the aircraft.

In step 60, the computer generates an output based on the decision takenin step 58 for supply to the control means 16 for implementing thedecision.

In this way, an anomaly in flight handling may be picked up andcorrected before the primary attitude sensors of the aircraft have evenbegun to provide “out-of-desired-flight-envelope” responses. Unexpectedor dangerous situations can thus be corrected before they develop intounplanned flight maneuvers.

The present invention provides a radical approach to flight control inthat it envisages a proactive arrangement based on excitation of theaircraft rather than a reactive system simply based on monitoring sensordata.

1. An aircraft control system having: means for detecting anirregularity in aircraft handling during flight; means, responsive to anoutput from the detecting means, for causing a temporary application ofa rigid body excitation in at least a portion of the aircraft; means formonitoring an actual response to the rigid body excitation; means foreffecting a comparison of the actual response with a target response ofthe aircraft to the rigid body excitation; and means responsive to anoutput from the comparison means for determining the need for amodification of the current flight plan for dealing with theirregularity and for generating a corresponding control output.
 2. Asystem according to claim 1 which includes means for calculating atarget response to the excitation.
 3. A system according to claim 2 inwhich the calculating means comprise a stored mathematical model.
 4. Asystem according to claim 3 in which the mathematical model is based onEuler Dynamic Equations of Motion.
 5. A system according to claim 3 inwhich the mathematical model is based on an analysis of aircraft controllaws and dynamic characteristics undertaken during design.
 6. A systemaccording to claim 1, in which the means for causing excitation comprisemeans for temporarily applying a low level periodic force to a rigidportion of the aircraft frame.
 7. A system according to claim 1, inwhich the monitoring means is arranged to detect a frequency/transientcharacteristic of at least a portion of the aircraft.
 8. A systemaccording to claim 1, in which the monitoring means is arranged todetect at least one of the following: inertial aircraft body rates,aircraft accelerations, and control surface positions.
 9. A systemaccording to claim 1, in which the comparison means is arranged togenerate a flight condition analysis as an output.
 10. A systemaccording to claim 1, in which the control output represents one of aflight control adjustment signal and a flight plan alteration signal.11. A method of controlling an aircraft comprising: detecting anirregularity in aircraft handling during flight; causing a temporaryapplication of a rigid body excitation in at least a portion of theaircraft in response to such detection; monitoring the actual responseof at least a portion of the aircraft to the rigid body excitation;comparing the actual response with a target response of the aircraft tothe rigid body excitation; determining the need for a modification ofthe current flight plan for dealing with the irregularity, andgenerating a corresponding control output.
 12. A method according toclaim 11 in which the rigid body excitation comprises a low levelperiodic force temporarily applied to a rigid portion of the aircraftframe.
 13. A method according to claims 11 in which the step ofmonitoring comprises detecting frequency/transient characteristics of atleast a portion of the aircraft.
 14. A method according to claim 11,which includes calculating a target response to the rigid bodyexcitation.
 15. A method according to claim 14 in which the step ofcalculating comprises employing a stored mathematical model ofanticipated responses.
 16. A method according to claim 11, in which thestep of comparing includes generating a flight condition analysis.
 17. Amethod according to claim 11, including the step of adjusting one of theflight control and altering the flight plan in response to the controloutput.